Listeria monocytogenes (Listeria) continues to represent one of the most significant foodborne pathogens. It is a bacterium, a microbe with devastating impact in susceptible people. Worldwide, the fatality rate of Listeria is 16% compared to 0.5% for the more commonly encountered food borne pathogen, Salmonella. This makes it one of the worst foodborne diseases of all because of its high mortality rates. In more recent years the fatality has been quoted at between 20 and 30 % (Thomas et al., 2015.
In the USA, Listeriosis causes 19% of the total pathogen-related fatalities from food poisoning (Scallan et al., 2011) for example. This amounts to 2,500 cases of food poisoning with 500 deaths annually in the USA (Mead et al., 1999). It is also an infection that produces an extremely high hospitalization rate (>90%) (Hernandez-Milian et al., 2014).
It can lead to meningitis, sepsis, miscarriage, premature delivery, and mother-to-fetus infections (Swaminathan et al., 2007; Schuppler and Loessner, 2010). It is able to withstand a variety of environmental stresses which makes it a much more potent food safety organism than many others that cause illness (Notermans et al., 1998).
The death rate from Listeriosis could be as high at up to 40% (Gandhi and Chikindas, 2007) and in those susceptible groups described, up to 75%.
Outbreaks are regularly recorded with 3 dying from contaminated Ricotta cheese (WebMD, 2012), deaths from Cantaloupe melons consumed in the US between 2011 & 2012 as reported in various news media, 17 cases of people eating contaminated camembert cheese, and 3 died (Johnsen et al., 2010) and of 8 cases with consumption of a Mexican cheese, where 7 pregnant Hispanic women fell ill (Jackson et al., 2011). Only strong antibiotic treatment will clear up the infection.
According to the Center for Diseases Control and Prevention, the reported incidence of listeriosis has remained unchanged in the past 10 years (CDC, 2014).
What Foods Does Listeria Contaminate ?
The foods most susceptible to Listeria contamination are ready-to-eat (RTE) foods especially meats and seafood. These are particularly high risk (Health Canada, 2011; Zhu et al., 2005).
Recent outbreaks have implicated sprouts, soft-cheese, hummus and apples confirming the continuing significance of Listeria. It is a major microbial contaminant of processed foods especially milk, dairy foods cheeses, butter, ready-to-eat (RTE) foods, pre-packed sandwiches, vegetables and fruit, fish, seafoods and certain processed meats such as frankfurters (Barza, 1985; Schlech, 2000; Gaulin et al., 2003; MacDonald et al., 2005; Varma et al., 2007; Jackson et al., 2011). It is commonly isolated from soils which means it often contaminates raw food materials (Beuchat and Ryu, 1997).
RTE meat products contaminated with Listeria are a high-risk food-pathogen combination especially because these foods do not undergo additional kill or cook steps and always need to be especially considered when constructing a HACCP plan (Schwartz et al., 1988; Gottlieb et al., 2006; Mataragas et al., 2008; CDC 2009). Processed foods such as chicken and turkey are also at risk (Barnes et al., 1989) as is cold salmon and other fish. Other typical foods are meat sausages such as the Portuguese delicacy Morcela de Arroz or the blood sausages of Majorca.
Growing Conditions For Listeria
It is a Gram-positive rod-shaped bacteria which is psychotrophic and halotolerant (Seeliger and Jones, 1986), anaerobic growing, facultative and non-spore forming (Larsson et al., 1999). These characteristics make it difficult to deal with without the appropriate processing steps during food manufacture and storage !
L. monocytogenes represents a great challenge to the food industry because of its ability to grow in a wide range of temperatures (0 to 45 °C), pH conditions (4.4 to 9.4), high salt concentrations, and persist on a variety of food-contact surfaces (Orgaz et al., 2013). It has a wider range for pH growth than any other bacteria. Some foods are especially susceptible. The consumption of RTE foods contaminated with L. monocytogenes by immunocompromised, elderly, newborn, and pregnant women who are most susceptible especially can lead to fatalities (Farber and Peterkin, 1991; Zhu et al., 2005).
Listeria Species
When setting microbiology specifications, Listeria always comes to the fore especially when examining ready-to-eat foods and dairy products. It is also a serious matter because of the litigation surrounding food poisoning events. There are six species; L. monocytogenes, L. welshimeri, L. ivanovii , L. seeligeri , L. grayi, and. L. innocua. By far the most serious because of its pathogenicity is L. monocytogenes. which causes the disease Listeriosis.
This particular bacterium has 4 evolutionary lineages (I, II, III, and IV) occupying overlapping ecological niches (Orsi et al., 2011). Currently, 12 serotypes are recognized for L. monocytogenes; 1/2a, 1/2b, 1/2c, 3a, 3b, 3c, 4a, 4b, 4c, 4d, 4e, and 7. Of these, the LM serotypes 1/2a, 1/2b, and 4b are responsible for over 98% of documented human cases of infection (Jacquet et al., 2002).
New strains of L. monocytogenes continue to be discovered which can pose a severe food safety threat. In 2019, international researchers from the Institute of Medical Microbiology at the Justus Liebig University Giessen (JLU) in Germany identified a new strain which could pose problems. This new strain was the cause of serious disease in sheep in a remote part of China, in the province of Jiangsu. This strain is hypervirulent. The researchers decoded the genome sequence so as to understand better what genetic factors were making this particular form so pathogenic. The virulence factors that make this particular strain especially dangerous were all combined in this single strain which made it unique.
The research illustrates the fact there are still strains of Listeria monocytogenes out there which are pathogenic and require urgent examination.
Regulations To Manage Listeria In Foods
(1) USA Regulations
There is a “zero tolerance policy” by the United States Dept. of Agriculture (USDA; 64 FR 27351) to Listeria because it has caused so many disease issues (Code of Federal Regulations, 2003). The USDA-Food Safety and Inspection Service (FSIS) and the Food and Drug Administration (FDA) regulatory policy requires a scientifically validated L. monocytogenes control program from processors producing RTE products (Gottlieb et al., 2006).
The FSIS Listeria Interim Final Rule (Listeria Rule) applies to any plant producing RTE meat or poultry products exposed to the environment after primary lethality treatments and cooking. The rule establishes three strategies or alternatives for controlling L. monocytogenes in RTE foods. An RTE processor can employ a post-lethality treatment and a growth inhibitor which is the best control or a less stringent second alternative where either post-lethality or a growth inhibitor is used. The third alternative uses only a sanitation program for controlling L. monocytogenes in the environment.
(2) European Regulations
The European Union (European Commission, 2005) has a limit of 100 cfu/g for ready-to-eat foods over the course of its shelf-life whether they can support its growth or not. These foods are not intended for infants or for other special medical purposes. The analytical reference method is EN/ISO 11290-2.
There must be an absence of the microorganism in a sample of 25 grams for RTE foods intended for infants and for specific medical purposes over the course of the shelf-life of the product. A similar limit is imposed for RTE foods not intended for infants too or for medical foods at the point before it leaves the food manufacturer. The analytical reference method here is EN/ISO 11290-1.
Processing Requirements
Listeria spp. are robust enough to grow at relatively low temperatures, around 4ºC as found in a refrigerator, tolerate high osmotic stresses and are able to protect themselves by forming biofilms. They generally do not like a pH below 5, but RTE foods are often above this pH.
Investigations have been on going over a number of years, see Doyle et al., 2001; in meats, (Johnsen et al., 2000). The review tabulates data for Listeria growth in a range of susceptible foods and should prove useful for food processors in determining appropriate times and temperatures for producing foods free of vegetative pathogens. It is clear that treatment conditions for Listeria must be more stringent than for other micro-organisms such as E. coli or Salmonella (Walls, 2006).
Cheeses are notoriously susceptible to harbouring Listeria if not cared for properly. Even though salt for cheese brining is often used as a microbial barrier and enhance food flavour, Listeria can survive quite severe conditions. Larsson et al., (1999) reported L. monocytogenes survival in commercial cheese brines up to 24% NaCl, for 259 days at 4°C.
Juice Processing – Minimal Processing
Some juice processors minimise Listeria growth by combining mild thermal treatments with changes in pH. For example, carrot juice can be produced safely if the juice is heated to 60°C for up to 5 minutes with the pH being adjusted to pH 4.5 usually with citric acid as it is not an acidic juice by nature. The juice then needs to be cooled rapidly to 4°C to maintain stability.
A similar processing practice was adopted with orange juice where its pH is 3.5 which is low anyway. In one study, there was a five-log cycle reduction of Listeria innocua in 6.2 min at 57°C or 1.7 min at 60°C. It is a good illustration of how an appropriate level of mild thermal treatment can make a significant impact in making juice safer generally.
Good Food Hygiene
To prevent listeria, always practise good food hygiene. Generally cross-contamination is the most likely method of seeing Listeria appear on a product. Studies are still underway to identify ways and means of controlling the micro-organism. The NHS has a very informative web-site about the disease and ways to prevent Listeriosis (NHS, 2012). There is also the Food Standards Agency web-site (FSA, 2012) which details strategies and approaches for dealing with outbreaks and the USDA compliance guidelines (USDA, 2006).
The Use Of Antimicrobials
Generally, there is increasing interest in the use of natural antimicrobials including benzoic and sorbic acid (Xi et al., 2011). In the study on orange juice production by Char et al., (2009) the use of mild heat treatment along with the addition of vanillin greatly improved the bacteriacidal effect of the mild heat treatment. The levels of natural antimicrobials to be added to each process is based on experimental assessment but there is a clear improvement in juice safety by their addition.
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